Development of wireless communication devices targeting for sensor networks has been increasing. Such wireless communication devices are generally required to have more than one-year of operating life on a small-capacity batteries, such as coin cell batteries. In order to extend battery life as long as possible, a low-power consumption communication protocols, such as Bluetooth® low energy (BLE), have been developed, as well as integrated circuit (IC) chips compatible with corresponding standards of the protocols have been developed as well. A device including an IC chip compatible with BLE standards, for example, operates intermittently by repeating a sleep state and an active state in a current consumption-time waveform, examples of which are shown in FIG. 12A and FIG. 12B. For example, a current on the order of several tens of milliamps (mA) is consumed during an active state (“connection event”) as shown in FIG. 12A, whereas current consumption is on the order of microamps (μA) or less, on average, during a sleep state (“sleep period”) as shown in FIG. 12A, which is extremely low. The operating clock in the IC is on the order of 1 microsecond (μs), and the time required for transition from the active state to the sleep state is on the order of 1 μs.
In such a device, the operation state inside the IC can be obtained by measuring current consumption profile. Measuring the current consumption profile, however, is challenging because the current draw changes by over four digits, e.g., from several tens of mA to about 1 μA in less than 1 μs.
A conventional method of measuring current in a device under test (DUT) uses a shunt resistor of 10Ω across input terminals of an oscilloscope with voltage probe, connected to the DUT, effectively measuring the current by dividing the voltage by the resistance. This is described, for example, by S. Kamath et al., Application Note AN092, “Measuring Bluetooth® Low Energy Power Consumption,” Texas Instruments Incorporated (2012), which is hereby incorporated by reference. A larger valued resistance works better for the measurement of low current on the order of 1 μA. In a simple example, a current waveform may be observed with a 1Ω shunt resistor and a sensing frequency bandwidth of 20 MHz. Assuming that the current is measured with the shunt resistor by a differential amplifier that has 1 nV/√{square root over ( )}Hz as input referred voltage noise spectral density, the root mean square (RMS) value of the current measurement noise is expected to be 1 nV/√{square root over ( )}Hz*√{square root over ( )}(20 MHz)/1 Ω=4.5 μA in root mean square (RMS).
Given this circumstance, it may be thought that the current waveform can be observed with noise on the order of μA or less by the choice of a 10Ω resistor. However, multiple bypass capacitors, e.g., approximately 1.5 μF in total, may be installed as parallel capacitors in a conventional measurement circuit in the DUT between a power supply voltage (Vcc) and ground (GND). To measure current draw from a coin cell battery to the IC and the bypass capacitors, a time constant made of the 10Ω shunt resistor and the 1.5 μF bypass capacitors becomes 15 μs, which is too long for the observation of the current draw change within 1 μs or less, thereby rendering the current waveform blunt. To avoid this situation, removing the bypass capacitors may theoretically enable faster current measurement, even with the 10Ω shunt resistor. However, a high speed IC may result in operation failure, and therefore measurement under a normal state operation may not be conducted.
As an alternative solution to avoid the long time constant, a shunt resistor may be inserted at the location of a power supply rail between the bypass capacitors and the IC. However, the resistance of the inserted shunt resistor can actually become an equivalent series resistance of the bypass capacitors, which is likely to cause operation failure in the IC. As stated above, fast current measurement should be conducted assuming that bypass capacitors having a capacitance on the above-mentioned order are connected between the Vcc and the GND.
The development of a wireless communication device, e.g., operating according to BLE or similar protocols, therefore requires current sensing means that can be used for observation of rapid current changes, and that has low noise with a wide current measurement range. It is therefore desirable for a current sensing means to have low noise performance that allows for an observation on the order of 1 μA, a wide dynamic range that allows for measurement ranging about five digits from about 1 μA to at least about 100 mA, a wide and flat frequency response from direct current (DC) to at least about 100 MHz, and fast response characteristics capable of tracking a current change that takes less than 1 μs when capacitance of a measurement circuit of the DUT is taken into account.